they were a cheap made fps that was popular when it first came out becuase the fps and tittes were a new thing in gamng at the time but it dont really have anything going for it these day's becuase they were never a great game to begin with.
DN3D was HUGE in the day. The number of mods, packs, maps, etc to be downloaded probably rivaled those from the Half-Life community. While a ton of the stuff was sheer and utter crap, not all of the best maps had nudity or any references to sex though the damn "shark in the tub" and toilet jokes abounded.

When DN3D was released, the comical references were reasonably current. People got the jokes. With DNF, the damn game had been in development for so long that even the references were lost. I played the demo on the PS3 and quite frankly... the references were so old it felt like the game was more appropriate for a five year old.
There are several reasons why DN3D was popular. For some, IIRC, DN3D had lower system requirements than Quake did. This was a big plus at the time at LAN parties because not everyone had upgraded to the Pentiums yet. When Quake brought support for OpenGL cards to the table, there were still those who
couldn't compete with the early adapters. Gameplay was too one-sided. That was also part of the reason why DOOM hung around for so damn long. Come to think of it, the people who were "amazed" at the pixelated nudity were those who never played DN3D before. Even in 1996, there were far better .gif porn to be had.
FPS pre-date DN3D. It wasn't exactly new, even then. Though I think the ability for a somewhat approximation of a proper Y axis was astonishing, I recall a few discussions about that. For a time, the Build engine was THE engine to use. I have a slew of titles with that crap engine. Redneck Rampage, Witchhaven, Shadow Warrior to name a few. The suck part is that Build never saw any improvements until Silverman released the source code. I didn't really see a lot of Quake based engines I guess until Half-Life came out two years later. By then I guess it didn't matter if I ran Build or Quake engines since I pumped up my PC with twin VoodooII cards.
I think one of the prime reasons why DN3D isn't popular today, even with new engines like eDuke is because the game simply didn't age well. When I play a game with resolutions so high I can pick out individual zits on the NPC's it's damn near impossible to go back to DN3D with the super pixelated titties. It's like driving a brand spanking new Toyota as a regular driver then going back and driving a Model T for nostalgia. It's fun a few hours I guess, but I like the speed and comfort of my daily driver thank you very much.